The present invention relates to cement compositions suitable for use in subterranean operations. More specifically, the present invention relates to cement compositions resistant to degradation in carbon dioxide containing zones and their use in subterranean cementing operations.
During the construction of a well, such as an oil and gas well, a hydraulic cement is typically placed into the annular space between the walls of the well bore penetrating a subterranean formation and the exterior surface of the well bore casing suspended therein. Such cement compositions have also been placed into the annular space between the walls of concentric pipes, such as a well bore casing and a liner suspended in the well bore. Following placement of the cement composition, further operations in the well bore, such as drilling, may be suspended for a time sufficient to permit the cement to set to form a mass of hardened cement in the annulus. The annular mass of hardened cement is referred to in the art as the “sheath.” The cementing procedure resulting in the initial construction of the sheath is often referred to as the primary cementing operation.
The function of a cement sheath may include providing physical support and positioning of the casing in the well bore, bonding of the casing to the walls of the well bore, preventing the movement of fluid (liquid or gas) between formations penetrated by the well bore, and preventing fluid from escaping the well at the surface of the formation. The set cement sheath should be able to endure a number of stresses during various downstream operations after the primary cementing operation.
In practice, a cement sheath may be compromised due to numerous stresses that may cause the cement sheath to fail resulting in a loss of hydraulic seal. In addition to physical stresses such as pressure and shear, conventional cementing materials may be susceptible to chemical alteration. For example, a typical hydraulic cement composition may suffer from carbonization in CO2 rich zones. Portland-based cements, in particular, may contain hydrated cement phases that may readily react with CO2 to form calcite, dolomite, and amorphous silica gel. Such chemical changes may negatively affect the porosity, density and texture of the cement sheath and may affect the sheath's mechanical and hydrologic properties. Moreover, such chemical degradation processes may compound problems arising from the physical stresses on the cement sheath, which in turn may compromise the sheath's hydraulic seal.
The hydraulic seal that the cement sheath provides may be particularly important in maintaining zonal isolation. If the seal becomes compromised, inter-zonal communication may lead to oil and gas flowing to lower pressure zones within the well rather than being directed into the wellbore for production. Loss of seal integrity may also lead to water production or annular pressure build up. Any of these occurrences may require expensive remedial services and/or may even result in the well being shut down in order to comply with regulatory procedures.